Spread spectrum communications system spread information over bandwidths much larger than those actually required to transmit the information. The spread spectrum technologies have been widely used both in military and commercial wireless communication systems such as the Global Positioning System (GPS), IS2000 mobile communications systems, and applications based on the emerging IEEE 802.15.4 standard. The advantages of using the spread spectrum approach are many. Spread spectrum systems are very robust with respect to noise and interferences due to the spreading gain. These systems are also inherently secure where multi-path fading has a lesser impact.
In the IEEE 802.15.4 standard, the transmitted data stream are grouped into 4 bits as a symbol and mapped and spreaded, i.e., encoded into 16 ary Psuedo Noise (PN) spreading codes where each of the 16 possible symbols is represented by a 32 bit PN code. Table 1 shows the 16 spreading PN codes defined by the IEEE 802.15.4 standard (“original PN codes”) where Symbol 0 through Symbol 15 is mapped into Codes 1 through 16. Each code is 32-bit long and represents 4 binary digits. The codes are designed to have the following properties: (1) Codes 2 to 8 are obtained by cyclicly shifting right 4, 8, 12, 16, 20, 24, 28 bits of the first code (symbol 0); and (2) Codes 9 to 16 are the inversion of odd-indexed chip value of codes 1 to 8.
TABLE 1Original PN Code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
In order to utilize the full potential of spread spectrum systems, spreading codes such as the original PN codes are constructed to have good auto- and cross-correlation properties. That is, with correlation during despreading, one code can effectively differentiate itself from the other codes under noisy conditions. FIG. 1 shows the cross-correlation properties of the original PN codes for the 2.4 GHz band between Code 1 and Codes 0-15. The first point represents the code correlating to Code 1 and its auto-correlation output. The correlation peak (FIG. 1, Code Index 1, value 32) appears when Code 1 correlates to itself. The second point represents the code correlating to Code 2 and its cross-correlation output and so on.
The spreaded PN codes are modulated using minimum shift keying (MSK) modulation schemes before transmission. MSK modulations schemes have many robust features and have been adopted in standards such as GSM, Bluetooth, and DECT.
A spreaded spectrum receiver has to demodulate and then despread the received signal with spreading codes. A typical implementation of these processes is shown in FIG. 2. Here, the received signal is first demodulated (22). Then, the demodulated signal is processed (24) using the spreading codes to generate the despread output by multiplying the demodulated received code with the spreading codes (26) and accumulating the products of the multiplication (28). The despread output is then used for bit or symbol decision-making or as input for error correction processing.
The receiver demodulation of MSK modulated signals can be either coherent or non-coherent demodulation. However, the receiver architecture of coherent demodulation is more complex and is more expensive to implement. In addition to data path match filtering, despreading and demapping, the coherent receiver requires carrier recovery and timing recovery circuits to determine the phase and frequency of carrier and timing clock in order to successfully recover the data stream. Therefore, coherent demodulation scheme of MSK signals are seldom adopted in industry due to the high cost in implementation.
The use of differential demodulation, a non-coherent demodulation, as the MSK demodulation scheme eliminates the necessity of carrier recovery circuits. Differential demodulation converts carrier frequency offset into DC offset thus requiring only DC removal circuits that are much simpler than carrier recovery circuits.
However, the unique properties of original PN codes are lost and changed after differential demodulation. The differential demodulated signals will require a new set of differential encoded PN codes for despreading with the use multiple correlators.
U.S. patent application Ser. No. 10/712,643 provides a novel receiver transformer that transforms the received demodulated signal. It also transforms the original PN sequence into a transformed differential encoded PN codes (“Differential Encoded PN Codes”) as shown in Table 2 for subsequent processing of the transformed received signal. With special setting on the initial condition of the differential encoded PN sequence, this Differential Encoded PN Codes have better properties than the original PN codes. The new codes have the following superior properties: (1) Code 2 (symbol 1) to Code 8 (symbol 7) are obtained by cyclicly shifting right 4, 8, 12, 16, 20, 24, 28 bits of the first code (symbol 0); and (2) Codes 9 (Symbol 8) to Codes 16 (Symbol 15) are the inversion of Codes 1 to 8. These new, superior properties allow a simple despeader/demapper implementation.
TABLE 2Differential Encoded PN Codes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
However, the despreading of Table 2 differential encoded PN codes is customarily accomplished using 16 correlators, one for each code of the differential encoded PN codes. Each received code of a demodulated transformed received signal has to be correlated with the 16 correlators to determine the maximum value of the correlation in order to determine the corresponding symbol for that received code. The hardware implementation of this method of despreading is very expensive and has to occupy a lot of ASIC area. In addition, this method of despreading requires the use of timing recovery circuits to address the timing shift caused by the sampling clock mismatch between the transmitter and the receiver.
Therefore, it is desirable to have innovative methods for despreading spread spectrum signals to overcome the shortcoming of prior art technologies.